The present invention relates to a method and apparatus in the treatment of the fiber fraction exiting from treatment of recycled fibers, and/or from vortex cleaning of a paper mill as mill reject, and/or in general a fraction exiting as a mineral-containing reject. As known, sources of recycled fibers include, for example, so-called broke of a paper machine, which may be recycled as raw material, and the actual post-consumer waste paper and board. The present invention especially relates to the treatment of recycled fiber materials of this kind in such a way that the fillers contained in the materials may be recovered as completely as possible and returned to the process as efficiently and economically as possible.
In the short circulation of paper machines manufacturing filler-containing paper grades and especially coated paper grades, a substantial amount of mineral and pigment fraction is discharged nowadays from the process as mill reject of a vortex cleaning plant which as a material could be utilized as raw material for paper but the particle size of this material is too coarse. Another source of mineral-containing rejects in a paper mill are filler and coating pigment manufacturing processes and the flushing waters from the apparatus used in these processes.
In the short circulation of paper machines manufacturing SC grades and other filler-containing grades, the mineral fraction exiting the vortex cleaning as mill reject is the coarse portion of the mineral fraction, i.e. the filler, supplied in the pulp dosing and usually having a particle size of over 10 .mu.m.
In the short circulation of paper machines manufacturing coated paper grades, the mineral fraction exiting the process as mill reject mainly contains non-dispersible coating layer of coated broke. The coating layer has not dispersed in the broke dispersion system to particles fine enough. This fraction of the coating layer, usually having a particle size of over 10 .mu.m, is rejected in the vortex cleaning of the short circulation.
The same applies also to systems for recycled pulp, in which coated magazine paper or like heavily coated raw material for recycled paper is used as the raw material. In the defiberizing system for recycled pulp the coating layer of the coated paper is detached from the fiber layer of the paper itself more of less in sheet-like fractions, which become partially crumbled in the process. The crumbling and dispersion are, however, not complete and thus, these non-dispersed coating pigment particles are discharged from the process as reject of the vortex cleaning in the screening stage of the recycled pulp line. However, in view of their raw material composition, a major part of these particles would be usable in the paper manufacture as filler which may have to be added to the pulp suspension at a later stage in the paper manufacture. The size of these particles, however, will cause problems in the paper machine itself if the particles are not dispersed and thus made usable as filler.
In an arrangement according to the invention, the filler/mineral loss exiting as mill reject from the vortex cleaning process and/or as reject from the manufacture of filler or coating pigment is reduced by treating the fraction having a concentrated mineral content in the vortex cleaning plant in order to disperse the mineral fraction and to return it to the process.
The method and the apparatus of the present invention provide for example the following advantages:
Filler/mineral, water, chemical, heat and fiber losses as small as possible. Only the useless fraction and the fraction that cannot be processed into a usable form are discharged in a highly concentrated form. PA1 Dispersion of mineral particles is based on internal shear forces of the suspension; in other words mechanical wear is minimal. PA1 Investment costs of the system are very low. The invention is technically carried out with commercial apparatus, repayment period is short. PA1 System is easy to build, i.e. it may also be established in existing systems by collecting mineral-containing rejects from various sources and by adding in the process, for example, subsequent to the last cleaner step or inside the cleaner plant, a screen to fractionate the mineral fraction and the organic fraction, and a cleaner of a new type from which the rejected flow is discharged in a highly concentrated form, and by adding in the process after these stages a treatment stage for dispersing the mineral fraction. PA1 The treatment is preferably carried out in the short circulation of a paper machine for each machine individually; thus for example the water circulations of the machines are not mixed. PA1 The system is a continuously operating part of a short circulation of a paper machine, or of a screening system. In other words, the process conditions are constant and the operation trouble-free. PA1 The process adjusts itself, for example, if the amount of coarse fraction increases, the system returns only the dispersed fraction to the process and the rest is discharged from the system.
The characteristic features of the method and the apparatus of the invention become apparent in the enclosed patent claims.
The method and the apparatus of the present invention are described more in detail below, by way of example, with reference to the accompanying drawings.